In your event's left navigation menu, click Event Website Builder. Under Available Blocks, find the block you want and click its + icon to add it to your website.

Click the cog icon on the block to open its settings menu. From there you can change anything about the block except the text it displays (see the next section for editing text).

Options like the section title and background color are edited in the settings menu, but the text itself is edited directly in the block preview on the right-hand side. Just click into the text to make your changes. The Cover block has extra settings options, but its text is edited the same way.

Under Added Blocks, find the block you want to remove, click the trash can icon next to it, and confirm the deletion.

If your event is published, you'll need to republish it for your changes to go live.